Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a charcoal canister. During a subsequent engine operation, the stored vapors can be purged into the engine where they are combusted. Various approaches may be used to generate vacuum for drawing in the fuel vapors. For example, an intake manifold vacuum generated during engine spinning can be used to draw in the stored fuel vapors. As another example, boosted intake air may be directly or indirectly used to purge the fuel vapors. Yet another example approach is shown by Ulrey et al. in U.S. Pat. No. 8,109,259. Therein, compressed air is directed through a crankcase to yield a crankcase effluent. Then, crankcase effluent is combined with the effluent from the canister which includes the stored fuel vapors. The combined effluent is then purged to the engine intake.
The inventors herein have recognized that such approaches may have limited performance during conditions when manifold pressure (or MAP) is at or near atmospheric conditions (or BP). In particular, during such conditions, the amount of vacuum available for purging the fuel vapors may be low, leading to a large vacuum valley. The reduction in the amount of purge vacuum available may lead to incomplete purging and degraded emissions. Further, in some examples, fuel economy may be sacrificed in order to increase vacuum for fuel purging, e.g. by forcing an engine re-start on an HEV by reducing use of variable camshaft timing or variable valve lift. Still other approaches may employ electric pumps for vapor purge in order to avoid this fuel economy penalty. However, such pumps may be expensive, and the electricity to power them may increase parasitic loads which degrade fuel economy.
In one example, some of the above issues may be at least partly addressed by a method for a boosted engine comprising: during boosted conditions, drawing vacuum at a first aspirator using compressor bypass flow. Then, during non-boosted conditions, the method includes enhancing intake manifold vacuum by drawing vacuum at a second aspirator using intake throttle bypass flow. Further, during both conditions, the method includes applying the drawn vacuum to purge fuel vapors from each of a canister and a crankcase to the intake manifold. In this way, one or more aspirators can be used to enhance low intake manifold vacuum and improve purging efficiency.
As an example, during non-boosted conditions, fuel vapors (from a fuel tank) previously stored in a canister may be drawn into an engine intake along with fuel vapors from a crankcase. In particular, both the canister vapors and the crankcase gases may be drawn into the intake manifold in a first, common direction using intake manifold vacuum. Optionally, the intake manifold vacuum may be enhanced (e.g., when manifold pressure is substantially at atmospheric pressure) by flowing at least a portion of intake air through an aspirator coupled in a throttle bypass and drawing additional vacuum at the aspirator. Alternatively, the intake manifold vacuum may be harnessed by flowing crankcase gases through an aspirator and drawing additional vacuum at the aspirator. In this way, throttle bypass flow is used to draw in the fuel vapors during non-boosted conditions.
During boosted conditions, fuel vapors from the canister and the crankcase may be drawn into a compressor inlet using vacuum generated at an aspirator coupled in a compressor bypass. Therein, both the canister vapors and the crankcase gases may be drawn into the intake manifold via the compressor inlet in the first, common direction. In this way, compressor bypass flow is used to draw in the fuel vapors during boosted conditions.
In this way, one or more aspirators coupled to an engine system may be advantageously used to provide additional vacuum for purging canister and crankcase fuel vapors. By using throttle bypass flow or crankcase flow to generate vacuum at an aspirator during non-boosted conditions, intake manifold vacuum can be enhanced during conditions when a large vacuum valley would otherwise occur. By using a compressor bypass flow to generate vacuum at a different aspirator during boosted conditions, the generated vacuum can be used to draw the canister and crankcase fuel vapors into the intake manifold while flowing the vapors in the same direction as during non-boosted conditions. The common handling of fuel vapors from the canister and the crankcase, as well as the unidirectional flow of the vapors during both boosted and non-boosted conditions reduces system complexity and enables component reduction benefits to be achieved without degrading purging efficiency. For example, a single oil separator can be used at the crankcase. By using an existing air flow to generate a purging vacuum at the aspirators, the need for dedicated vacuum pumps is reduced, reducing related parasitic loads. Overall, emissions performance is improved without reducing fuel economy.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.